1. Field of the Invention
The present invention relates to an embedded type semiconductor laser which can be manufactured by selective diffusion, and a method for manufacturing the same.
2. Description of the Prior Art
A semiconductor laser is being applied to the fields of optical communications and optical information processing as a small-size and light-weighted light emitting source.
For a semiconductor laser, mode control of the guided light is necessary, and in particular, control of the transverse modes is important. Various kinds of laser structure have been proposed so far for controlling the transverse modes of the semiconductor laser. Among these it has been found that the embedded type semiconductor laser is essentially effective.
As to the method of manufacturing embedded type semiconductor lasers, proposed structures may be classified roughly into the following three categories. The first is a method in which crystal growth that includes an active layer, and mesa etching that excludes a required width of the active layer, are carried out first, followed by an embedded crystal growth on the side etched surfaces of the mesa. The second is a method in which formation of grooves and ridges on the semiconductor is carried out, and then a selective crystal growth is carried out so as to embed the active layer by the groove and ridge portions that are formed. The third is a method in which crystal growth which contains an active layer is carried out first, and then impurity diffusion is carried out leaving a required width of the active layer to lower the refractive index of the side surfaces. In the third method, and in particular when the active layer is so thin as to form a quantum well, the effect of interdiffusion of atoms at hetero-interface due to impurity diffusion becomes conspicuous, so that it becomes possible to cause a selective modification of the composition.
A comparison of the above three methods shows that in the first method, the thickness of the active layer, the embedding width, and the refractive index of the embedded crystal (crystal composition) can be selected independently, so that the degree of freedom in device design is high. However, in the second crystal growth for embedding, the side surfaces of the active layer are exposed directly to the high temperature gas so that the quality of the crystal interface tends to be deteriorated. There also remains thermal distortion due to crystal re-growth.
In the second method, although problems such as those appear in the first method can be resolved, the controllability of the film thickness and the width of the active layer is low due to the use of singular crystal growth in the groove and the ridge portions. In the third method, crystal growth is essentially only for once, and the crystal growth for embedding for the second time is unnecessary in principle. However, it has a drawback such as the difficulty in the impurity diffusion for embedding.
The present method relates to an improvement of the third method. In the third method, drawbacks such as those exist in the first and the second methods are nearly eliminated, but there is a drawback in that it has a difficulty in manufacturing, as mentioned above. In what follows, the prior art of the third method will be described. In so doing, reference will be made to a mixed crystal system of Al.sub.x Ga.sub.1-x As as an example.
In FIG. 1 is shown an example of impurity diffusion type embedded semiconductor due to the prior art. Reference numeral 501 is a GaAs substrate, 502 is an Al.sub.x Ga.sub.1-x As cladding layer, and 503 is a quantum well active layer due to Al.sub.y Ga.sub.1-y As/Al.sub.z Ga.sub.1-z As which is a multi-quantum well in this example. Reference numeral 504 is an Al.sub.x Ga.sub.1-x As cladding layer, 505 is a GaAs ohmic contact layer, and 506 is an impurity diffusion region where Zn, Si, S, and others are used as impurity. Reference numerals 507 and 508 are electrode metals. The region where 503 and 506 overlap is the region where the quantum well structure is disordered due to impurity diffusion, the so-called disordered region. In the disordered region, it is possible to lower the index of refraction compared with the region which is not disordered so that it is possible to form an embedded wave guide. Problems in such prior art are the formation method and the profile control of the impurity diffusion region 506. Namely, in the prior art as shown in FIG. 1, impurity diffusion in vapor phase is carried out by providing a stripe-formed selective diffusion mask. In doing so, it is necessary to precisely control the partial pressure of diffused impurity for controlling the diffusion depth, and to control the temperature and the like. In addition, in the prior art of FIG. 1, a diffusion depth of greater than 2 [.mu.m] is necessary and a sufficient consideration on the diffusion profile is necessary. That is, it is due to this circumstance that the diffusion profile form tails as shown in FIG. 1 so that the variations in the diffusion depth to the variations in the width of the active layer to be embedded becomes large. In the prior art, considerations to prevent the influence due to the diffusion profile were not given.
As in the above, in the prior art there were problems in the areas of controllability and reproducibility of the width of the active layer with respect to the diffusion depth in the manufacturing process.